There are lots of reports about radar technologies that can penetrate stealth, since years. All you need is a radar system that does not rely on its own sender. It can work with mobile tower signals, the new Eurofighter radar will work with TV station signals afaik. It also works with multiple planes flying in formation, detecting the radar reflection from the other plane's senders. The Serbian radar system just used a longer wavelength than usual, that was already enough to penetrate stealth of the F117. Upgrading older planes with stealth-penetrating radar is also much cheaper than building a fleet of stealth fighters.